This invention generally relates to electron beam lithography systems, and more specifically, to methods and apparatus for positioning a wafer in such systems.
Electron beam lithography systems are used to make large scale integrated circuits. A principle advantage of these systems is that they may be used to manufacture extremely fine lines. For example, electron beam writing systems may be used to form lines having widths of less than 0.05 xcexcm with an alignment tolerance of less than 0.02 xcexcm.
In the operation of an electron beam lithography system, an electron beam is generated and directed through one or more masks or reticles that shape the cross section of the beam into a desired pattern. The shaped beam is then directed onto a wafer to form, or write, a pattern on the wafer, and typically the cross-sectional pattern of the beam is transferred into an electron sensitive polymer layer on the top surface of the wafer. That pattern may be formed in one exposure of the wafer to the electron beam, a procedure referred to as xe2x80x9cone shot. xe2x80x9d
The surface of the wafer must be placed in the correct z-plane prior to exposure to insure that images are projected from the mask to wafer xe2x80x9cin focusxe2x80x9d. The time to measure and adjust focus must be made small to achieve high exposure tool parts per hour capability. Current techniques for determining the z-location of the wafer surface are optical reflectance, capacitive gauges, and air gauges. These techniques can be influenced by the presence of underlying films, and may not repeatedly measure the top surface location.
For instance reflectance measurements may indicate a different z-position of the top surface, when in fact all that has changed is the film stack on the wafer. Capacitive gauges are inaccurate at wafer edges due to fringing of electrical fields as the gauge moves off the wafer. Also, capacitive gauges must be calibrated for each film stack combination to obtain absolute values of wafer surface position. This is because the capacitive measurement is sensitive to material thickness and electrical properties of the stack. Air gauges have bandwidth and response time limitations which may limit exposure tool speed. Air gauges sometimes introduce unwanted contamination.
An object of this invention is to control wafer z-axis position accurately in lithography exposure tools.
Another object of the present invention is to use a scanned probe microscope or SPM to control the z-axis positioning of a wafer is an electron beam lithography system.
A further object of this invention is to provide a method and system, using an SPM, that may be effectively employed in an electron beam lithography system to determine the z-axis positioning of a wafer over various types of topography.
These and other objectives are attained with a method and apparatus for positioning a wafer in an electron beam lithography system. This method includes the steps of positioning a scanned probe microscope in the lithography system, and determining the distance between a preset location on the scanned probe microscope and a reference position in the lithography system. The wafer is brought into physical contact with that preset location, and then the wafer is moved a predetermined distance from the preset location on order to position the wafer at the desired focal plane in the lithography system.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.